Passivity zone

A more recent advancement of AP has come from the application of a controlled cathodic cnrrent which can be utililzed to shift the corrosion potential back to the passive zone. This (refinement) technique is usually termed the cathodic potential adjustment protection (CPAP). 

Characterization of the cavitational phenomena and its effects in sonochemical reactors are generally described through mapping. Mapping of sonochemical reactor is a stepwise procedure where cavitational activity can be quantified by means of primary effect (temperature or pressure measurement at the time of bubble collapse) and/or secondary effect (quantification of chemical or physical effects in terms of measurable quantities after the bubble collapse) to identify the active and passive zones. 

All specimens anodlcally phosphated were Initially polished to 600 grit, cleaned and Immersed In the phosphatlng solution Oxy-Plus 84 DRS, before applying a potential of +0.8V/SCE. This potential corresponds to the passive zone In the cyclovoltammogram obtained for the test steel In the phosphatlng solution. ... 

The thermodynamic information is normally summarized in a Pourbaix diagram7. These diagrams are constructed from the relevant standard electrode potential values and equilibrium constants and show, for a given metal and as a function of pH, which is the most stable species at a particular potential and pH value. The ionic activity in solution affects the position of the boundaries between immunity, corrosion, and passivation zones. Normally ionic activity values of 10 6 are employed for boundary definition above this value corrosion is assumed to occur. Pourbaix diagrams for many metals are to be found in Ref. 7. 

Figure 16.16 is a representation of Fig. 16.1 a in terms of the type of reaction it shows, as a function of pH, the zones where corrosion by dissolution occurs, where initial corrosion forms insoluble oxides on the surface that impede further reaction (passive zone) and the region where the metal is stable (immune zone). 

Oxidants such as nitrite and chromate which function by shifting the the surface potential of the metal in the positive direction until the passive zone in Fig. 16.6 (note that if these components are present in insufficient quantity the metal stays in the active zone, with potentially disastrous consequences). 

Taking into account that tvt = l/ vx/ o is the time of VT relaxation, the effective reaction time in the passive zone is much shorter than the relaxation time (tp tvt)- The strong non-equilibrium (Tv > To) can be sustained during the entire period of the effective... 

Integration of eqrration (6-55) gives a characteristic time tchem of NO destraction in the passive zone with respect to total drrration Tp (6-46) of the effective synthesis outside of the active zone ... 

The produced nitrogen oxides are stabilized in the passive zone if rp/Tchem 1. To stabilize NO in the active and passive zones ((6-53), (6-56)), it is necessary to have n/ 2 < 1 To suppress reverse reaction (6-52), atomic nitrogen should preferentially react not with NO but with O2, propagating the Zeldovich chain. Reverse reaction (6-52) is fast and has no activation barrier whereas the direct reaction (6-3) has an activation barrier of about... 

Plasma-Chemical NO Synthesis from Air Outside of the Active Discharge Zone (in Passive Zone). NO synthesis stimulated by vibrational excitation continues outside of the active discharge zone, using previously accumulated vibrational energy. The effective chemical process continues in the passive zone during a time interval Tp until the vibrational temperature from an initial value decreases to the critical value Based on (6-45), derive relation (6-46) for tp. 

The potentials that indicate the susceptibility to SCC can be determined by the scanning of potential-current curves at different scan rates. An example for carbon steel is shown in Figure 1.20. Potentiodynamic polarization curves involve the recording of the values of current with changing potentials (scan rate 1 V/min). This simulates the state of crack tip where there is very thin film or no film at all. To simulate the state of the walls of the crack, a slow sweep rate of lOmV/min is needed such that the slow scan rate permits the formation of the passive oxide film. The intermediate anodic region between the two curves is the region where SCC is likely to occur. This electrochemical technique anticipates correctly the SCC of carbon steel in many different media. The polarization curves also show the active zone of pitting and the stable passive zone before and after the expected zone of SCC susceptibility, respectively. 

In this paper, a surcharge loading test on a soil nailed slope model was conducted. The Pulse-PrePump Brillion Optical Time-Domain Analysis (PPP-BOTDA) technology was used to monitor the strain distributions the slope model. During model construction, a special tight-buffered optical fiber was embedded in the soil mass and served as fully distributed strain sensors. The horizontal strain patterns at the active and passive zones... 

With the increase of load on the slope crest, the strains of each layers increased accordingly, which has a good correlation with the load increments. At all levels of load, the horizontal strains near the two segment ends were small and the strain accumulated near the central part. There were strain concentration phenomena adjacent to the slope crest (H3 layer). Strain inhomogeneity was very obvious when the critical condition of slope stabihty came close. The uneven strain distributions reflect that shear strains accumulated within the soil mass and the slope slip surface gradually formed. In this case, the strains of the passive zone... 

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